1. Field Of The Invention
This invention relates generally to amplifier circuits and more particularly to an amplifier circuit having both offset voltage correction and offset voltage drift correction.
2. Prior Art
Offset voltage amplifiers are known and have been used. In particular such amplifiers have used techniques which correct for offset voltage of a differential amplifier. Such techniques are varied in that some require a circuit coupled to the amplifier for sensing and storing of the offset voltage during a selected time period and supplying it to the amplifier input so that it is in opposition to the offset voltage during a second time period when the amplifier is connected to the signal source thus cancelling the offset voltage.
One technique is shown in U.S. Pat. No. 3,988,689. This patent teaches a circuit for cancelling the offset voltage of a signal amplifier using first and second amplifiers together with a third amplifier coupled to the outputs of the first and second amplifiers. The feedback from the third amplifier to the inputs of the first and second amplifiers is such that offset voltages are stored in a capacitor connected between the inputs of the second amplifier. When offset voltages are so stored on the capacitor, switches are opened to permit signal amplification through the signal amplifier to cancel the offset voltages.
A different technique is shown in U.S. Pat. No. 4,429,281 which describes an integrated circuit in which the outputs of the operational amplifier are coupled back, through a capacitor, to the input of the device in order to prevent potentials at the input and the output from varying and to ensure rapid stabilization of the integral operation.
Still another is shown in U.S. Pat. No. 4,229,703 which teaches the use of a zero reference in an offset compensation circuit where the output amplifier has a feedback loop comprising a resistive capacitive combination.
Finally, U.S. Pat. No. 4,190,805 teaches a commutating auto zero amplifier having a resistive feedback loop which can be coupled directly to either the negative input of the amplifier or capacitively to the positive input of the amplifier.
The above described techniques do not, especially with new, faster, higher frequency FET circuits, provide offset voltage correction and offset voltage drift correction. Especially they do not provide such corrections with low offset error nor do they operate without large complex matched transistor circuitry.